This invention relates to a polarographic electrode assembly suitable for transcutaneous measurement of partial oxygen pressure in arterial blood. A measurement of a partial oxygen pressure in arterial blood is an important matter in breath control of a new-born baby or a patient in intensive care.
Hitherto, for measuring the concentration of oxygen, namely the oxygen partial pressure PO.sub.2, in blood, the method most generally used has been that of measuring oxygen in the blood extracted from an artery. But, the method is not suitable for continuous measurements, and moreover, the method has the problem of being painful for the patient. Especially for a new-born baby, breath control is necessary in order to prevent an impediment of its brain due to low oxygen concentration while preventing damage in its retina due to excessive oxygen concentration. For such new-born baby, the oxygen concentration in the atmosphere surrounding the baby must be carefully controlled by using the measured value of oxygen partial pressure in arterial blood, and for that purpose a continuous or real-time measurement of oxygen partial pressure is necessary. A method of retaining an oxygen electrode in an artery for the continuous measurement can be considered, but this method requires a high handling skill and is likely to invite a great danger. Therefore, such a method has not been widely used.
The transcutaneous oxygen electrode method, being different from the direct measurement method, does not give the patient any pain. The method is suitable for long continuous measurement, since it catches oxygen at the skin surface and measures the oxygen which diffuses from the blood and through the skin. The transcutaneous oxygen electrode is essentially a kind of Clark-type electrode, but the former has a constant temperature-heating means to warm the patient's skin for arterialization. When it is attached on the patient's skin, oxygen diffusing from the arterialized subcutaneous tissue reaches the surface of a cathode of noble metal through an electrode membrane disposed between the skin and the electrodes. Then the oxygen reacts with the cathode and is reduced to water. By measuring the electrolytic current produced by this electrode reduction, the oxygen partial pressure PO.sub.2 can be obtained. In such measurement, by heating the part of the skin that contacts the electrode to a suitable temperature, the subcutaneous tissue is locally arterialized, thereby making the oxygen partial pressure to be measured practically equal to that of the arterial blood.
There are two representative known types of transcutaneous arterial blood oxygen measuring electrode assemblies, but these known ones have respective shortcomings and hence are inconvenient in handling. Especially, stable fixing of the electrode members is very difficult and their measured values are not sufficiently stable.
One of these types is the anode heating type which has a very fine needle cathode supported coaxially in a thick tubular anode which is heated at a constant temperature of 43.degree.-44.degree. C. Heating of the skin for local arterialization is carried out by this anode with an insulating material filled therein.
The construction of the anode heating type electrode assembly is elucidated referring to FIG. 1. As shown in FIG. 1, a few fine platinum wires 1 of about 0.015 mm diameter for cathodes are disposed in an insulating glass cylinder 3, and a cylindrical anode 2 of silver is disposed to surround the glass cylinder 3. And an oxygen-permeable membrane 4 of 12 .mu.m Teflon (trademark), PTFE is attached on one end face of the anode 2 and cathodes 1 and fixed by an O-ring 9 on a side face of an electrode holder 7 of plastics. A heater 5 is disposed around the anode 2 so as to heat the anode 2 to a constant temperature by the aid of a temperature detecting unit 6 in the glass cylinder 3. A collar 8 of plastics fixed to the electrode holder 7 has a contact face which is to be secured by double-side adhesive tape 12 to the skin 13 of a patient. A small amount of an electrolyte 11 is retained in a thin space between the end faces of the electrodes and the membrane, and another amount of the electrolyte is in a reservoir space 10 which is connected to the thin space. Also, a small amount of contact liquid 14 is filled between the membrane 4 and the skin 13.
The abovementioned anode heating type electrode assembly has the following problems. Due to the very small area of the working surfaces of the cathodes, it gives a good correlation coefficient of the measured value over actual arterial value, but by the same reason it has poor signal to noise ratio (S/N ratio). Secondly, since the plane membrane 4 is fixed by O-ring 9 on the side face of the cylindrical electrode holder 7, the membrane face is likely to have wrinkles and membrane stretch is not uniform and stable. Therefore, a uniform contact of the membrane 4 with the working surfaces of cathodes 1, which is necessary for stable measurement, cannot be fully expected. Thirdly in order to obtain partial oxygen pressure of the arterial blood by heat-arterialization of the subcutaneous tissue, this type of electrode assembly heats the skin by silver anode through the electrolyte layer and the electrode membrane, and the heating condition is not very stable.
The principle of the transcutaneous measurement of the partial oxygen pressure by using the anode heating type electrode assembly is elucidated referring to FIG. 1. Then the electrode assembly is stuck on the skin 13 of a patient by the double-face adhesive tape 12 at the end face of plastic collar 8 and the anode 2 is heated to 43.degree.-44.degree. C., the skin tissue at the part under the anode 2 and its neighboring skin tissue are heated thereby arterializing the skin tissue. Therefore, the partial oxygen pressure in the blood vessel in the heated skin tissue become substantially equal to that in the arterial blood. The oxygen diffuses from the blood vessel through the skin tissue, passes the membrane 4, dissolves in the electrolyte 11 consisting mainly of KCl solution and reaches the surface of the cathode 1. When a specified D.C. potential of 0.5 to 0.8 volts is applied across the cathode and the anode in a manner that the anode is positive to the cathode, by reaching of oxygen to the electrode surface, a reduction reaction of the oxygen takes place at the cathode surface, and an oxidation reaction of the silver takes place at the anode. Namely, on the surface of the cathode of gold or platinum, the reduction reaction is, in case that the electrolyte is acidic: EQU O.sub.2 +4H.sup.+ +4e.fwdarw.4H.sub.2 O . . . (1),
or, in case that the electrolyte is basic: EQU O.sub.2 +2H.sub.2 O+4e.fwdarw.40H.sup.-. . . (2).
In both of the above reactions, electrons of a number proportional to the amount of the O.sub.2 molecules reaching the cathode are consumed.
At the same time, on the surface of the anode 2 of silver, the oxidation reaction is, for any value of pH: EQU 4Ag+4Cl.sup.- .fwdarw.4AgCl-4e . . . (3),
thus electrons of the number corresponding to the amount of the O.sub.2 reaching the cathode are produced. Accordingly, a current flows between the anode and the cathode, and the intensity of the current is proportional to the number of oxygen molecules which pass through the membrane and hence is proportional to the partial oxygen pressure in the subcutaneous tissue and to the arterial blood.
A second known transcutaneous oxygen electrode assembly is a cathode heating type having a disk shaped working surface of cathode disposed coaxially in a tubular anode. One of this type of the prior art is shown in FIG. 2. A cathode of the electrode assembly consists of a platinum cylinder 1' of a large diameter having a backing copper block 1", and heater winding 5 is located in an encircling recess which is provided around the copper block 1" so as to directly heat the cathode 1. An anode 2 consists of silver ring coaxially disposed around the cathode 1 with an insulator or electrode holder 7 in between. An electrode membrane 4 is a thin film of plastics (e.g. Teflon) disposed on the end faces of the cathode 1 and the anode 2 with electrolyte 11 in between, and the membrane 4 is secured by a substantially annular plastic collar 8. An actual electrode assembly of the cathode heating type has the construction as shown in FIG. 2(b), wherein cathode 1 has such a large diameter as 3 mm, and the membrane 4 is 6 .mu.m thick polyester film held by inserting and pinching at its periphery between the annular capsule 8 and the cylindrical electrode holder 7.
The electrode assemblies of FIG. 2(a) and FIG. 2(b) have the following problems. Firstly, although a large area for cathode is selected, this area is not enough for the purpose of heating the skin, because the cathode is disposed in the anode whose size is limited for stable membrane fitting. In addition to this the skin is heated by this cathode via the electrolyte layer and membrane, and hence the heating of the skin tends to be insufficient. Secondly, since the cathode has a large area, the amount of the electrolytic reaction and hence the consumption of the electrolyte held becomes large, and this causes a large drift of the electrode sensitivity and a short lifetime for the electrode. Thirdly, since the cathode surface has a large round area, the distances to the anode and hence the exchange of the electrolytes are not equal in the central part and in the peripheral part. Therefore, the overall current of the cathode reaction does not precisely respond to the change in the partial oxygen pressure. Fourthly, since the large working surface of the cathode consumes a large amount of oxygen, the oxygen flux in the skin tissue becomes large. This causes the skin to disturb the flow of oxygen, and hence the partial oxygen pressure is measured as a smaller value than in actually is, in the other words, a poor correlation coefficient of the measured value over the true value is obtained. Such measurement error can be reduced to some extent by using an electrode membrane of relatively low oxygen permeability, for example, 6 micron thick polyester film. However, use of such a membrane results in a slow response in measurement.
Fifthly, since the electrode membrane 4 is secured by pinching the peripheral part by inserting it between the electrode holder 7 and the collar 8, the membrane is likely to have wrinkles, and tension on the membrane is not stable, thereby resulting in unstable sensitivity of the electrode.
In both of the conventional devices of FIGS. 1(a), 1(b) and FIGS. 2(a), 2(b) stretching tension of the membrane is not strong due to the loose fixing methods, and in addition to this, a wide range of the membrane area is exposed to skin. Accordingly, even a small change in the contacting condition of the electrode membrane to the skin is likely to make a change of the thickness of the electrolyte layer and hence changing the electrode sensitivity and making the measurement unstable.